[Manila CSI] add csi-sanity tests

[gman0]

This is a tracker ticket for PRs needed to get csi-sanity tests in csi-manila.

I'm working on this.

[gman0]

/kind feature

[gman0]

4 pending/to-be PRs:

• add an abstract interface for the Manila client, move it into a separate package, change all references from gophercloud.ServiceClient to manilaclient.Interface:
  feature branch https://github.com/gman0/cloud-provider-openstack/tree/csimanila-manilaclient
  PR [csi-manila] Manila client moved into a separate package #716
• add interfaces for the CSI client, move them into a separate package, change all references from csi.Identity/NodeClient to csiclient.Node/Identity
  feature branch https://github.com/gman0/cloud-provider-openstack/tree/csimanila-csiclient
  PR [csi-manila] CSI client moved into a separate package #728
• fix the driver in accordance with the CSI specification in order to pass the csi-sanity test suite:
  feature branch https://github.com/gman0/cloud-provider-openstack/tree/csimanila-csi-spec-compliance-fix
  PR [csi-manila] fixed incorrect behaviour discovered by csi-sanity test suite #730
• implement fake Manila client based on the manilaclient.Interface interface, add sanity tests:
  feature branch https://github.com/gman0/cloud-provider-openstack/tree/csimanila-sanity

Even though the latter three bullet points don't have a PR yet, they are finished and ready to go ASAP - as soon as the PR that's currently pending is finished with reviews and gets merged.